THE Pd–Ni–Fe–s Phase System At 550 and 400 °C

The condensed Pd–Ni–Fe–S phase system was studied at 550 and 400 °C by synthesis in evacuated silica glass tubes and a combination of textural and electron microprobe studies. The monosulfide Fe1–xS–Ni1–xS solid solution persists at both temperatures. Maximum solubility of Pd, exhibited at elevated x values in the mss formula, is 0.7 at.% at 550 °C and up to 0.1 at.% at 400 °C.

The field of sulfide melt is reduced to a narrow Ni-Pd rich stripe at 550 °C and is absent at 400 °C. Sulfur contents of the melt are limited to the interval 30–40 at.% S at 550 °C, i.e., it is metal-rich. Retreat of the melt field at 550 °C results in a plethora of phase associations in its place. Compositional fields of alloys associated with melt and of those associated with ∼(Ni,Fe)3±xS2 extend to high Pd contents.

Several sulfur-poor associations contain alloys with only low nickel contents, close in composition to Pd3Fe. Shrinkage of the ∼(Ni,Fe)3±xS2 phase field (the β1-β2 pair of Kitakaze et al. 2011) and the appearance of pentlandite represent the most important developments in the phase system between 725 and 400 °C.

The ∼(Ni,Fe)3±xS2 phase dissolves up to 2 at.% Pd at 550 °C. Pentlandite associated with Pd-rich phases dissolves up to 4 at.% Pd at 550 °C, and 5.4 at.% Pd at 400 °C. The latter value represents almost 92% Pd occupancy of the octahedral site. At 550 °C, Pd-rich pentlandite coexists with sulfide melt which contains between 17 and 47 at.% Pd.

The rare occurrences of such Pd-rich pentlandite in nature could indicate the presence of Pd-rich residual melts. Other cases of Pd-rich pentlandite associate with Pd-rich alloy or phases like PdS and Pd2.2S. In the association with mss, Pd is concentrated in the sulfide melt, especially at a low temperature.

At low temperatures or high S fugacities, nickel prefers mss but pentlandite and ∼(Ni,Fe)3±xS2 complicate this simple scheme. For mineral associations with elevated sulfur fugacities, pyrrhotite (mss) appears to be a principal solid-state collector of Pd at high temperatures, with its gradual release on cooling, making it available for subsequent reactions.

At low sulfur fugacities Pd concentrates in its Fe-Ni alloys. Because of the inverse relationship between Ni solubility in PdS and the temperature of formation, the composition of natural vysotskite (and Pd-rich braggite) may serve as a temperature indicator.